|Glaciers & Ice Ages|
Louis Agassiz, a Swiss geologist, recognized that glaciers explained erratic boulders. He observed that glaciers were powerful agents of landscape change.
Agassiz proposed that ice sheets had once covered Europe, had sculpted the landscape, and had retreated, leaving behind fine-grained unsorted soils and erratic boulders.
Agassiz’s idea was criticized for decades, but finally
accepted; the evidence was simply too strong. Agassiz saw evidence for
an ice age in North America, too.
Glaciers presently cover ~10% of Earth. During ice
ages, this coverage expands to ~30%.
The most recent ice age ended ~11 Ka. New York,
Montreal, London, and Paris were buried below hundreds to thousands of
meters of crushing, grinding ice.
Ice and the Nature of Glaciers
What is ice?
Snow falls like sediment and accumulates in layered strata.
Layers of snow recrystallize to become metamorphic
Conditions Needed for Glacier Formation
How a Glacier Forms
Under a microscope, glacial ice has coarse grains and contains air bubbles.
Air content decreases with age and degree of metamorphism.
Snow compacts and melts to form firn, which recrystallizes into ice.
Crystal size increases with depth.
Categories of Glaciers
Piedmont glaciers spread out at the base of a mountain
Two major continental ice sheets exist
today—Antarctica and Greenland.
The Movement of Glacial Ice
Ice flows downhill under the influence of gravity. The
ice base can flow up a local incline.
Ice flows away from the thickest part of continental
glaciers; analogous to honey flowing away from the thickest part of
The Movement of Glacial Ice
The equilibrium line separates the zone of accumulation from the zone of ablation.
Ice flows downward in the zone of accumulation and upward in the zone of ablation.
Glacial Advance and Retreat
The position of the toe represents a balance between
addition by accumulation and loss by ablation.
If accumulation exceeds ablation, the glacier advances, the toe moves farther downvalley, and the ice thickens.
If ablation exceeds accumulation, the glacier retreats and thins. The toe moves upvalley, even though ice continues to flow toward the toe.
Ice in the Sea
Marine glaciers are grounded in shallow water and
float in deeper water. Floating ice is mostly (80%) beneath the
waterline. Icebergs form when the leading edge of the glacier breaks
Floating ice exhibits a variety of shapes and sizes. Icebergs are >6 m above water. Ice shelves produce large tabular bergs.
Calved icebergs are laden with sediments eroded by the glacier and incorporated into the ice.
Icebergs raft sediments away where they are eventually released by melting.
Dropstones in marine muds are an indication of
Sea ice is non-glacial ice
formed from frozen seawater. Large areas of polar seas are covered with
Glacial erosion produces deep, steep-sided valleys and
jagged, knife-edged ridges and pointed spires.
Rock fragments embedded in glacial ice act like
sandpaper on underlying bedrock. The moving ice abrades and polishes
substrates, producing a fine pulverized “rock flour.”
A roche moutonnée is an asymmetric bedrock hill shaped
by the flow of glacial ice. Abrasion rasps the upstream side, and
plucking carries away fracture bounded blocks on the downstream side.
Glacial Erosional Landscapes
Before glaciation, valleys are V-shaped, and tributary
mouths are the same elevation as the trunk stream.
A horn is a pointed mountain peak formed by three or
more cirques that coalesce.
The Matterhorn in Switzerland
An arête is a knife-edge ridge formed by two cirques that have eroded toward one another.
A cirque is a bowl-shaped basin formed at the uppermost portion of a glacial valley.
Aggressive freeze-thaw chews into the cirque headwall.
After the ice melts, a cirque is often filled with a
Glacial erosion creates a distinctive U-shaped trough valley.
These are easily discerned from V-shaped fluvial
A hanging valley results from the intersection of a
tributary glacier with a trunk glacier.
The larger trunk glacier incises much deeper into the bedrock than the smaller tributary glacier.
When the ice melts, the troughs have
different elevations and a waterfall results.
Fjords are U-shaped glacial troughs that have become
flooded by the sea.
Glacial Deposition: Transport of Sediment by Ice
Glaciers act as large-scale sediment conveyor belts. Sediment falls onto a glacier and gets plucked up from below. This material is transported to the toe, where it piles up as an end moraine
Glaciers are dirt machines; they carry an enormous
amount of sediment.
Lateral moraines form along either side of a valley
glacier. Medial moraines occur in the middle of a valley glacier and
result from the merging of two lateral moraines.
Glacial till is unsorted,
unstratified sediment dropped by glacial ice. Till is made up of all
grain sizes, from boulders to clay. It accumulates beneath glacial ice,
at the toe of a glacier and along glacial flanks.
Types of Glacial Sedimentary Deposits
Glacial erratics are cobbles and boulders that have
been dropped by a glacier, often on glacially polished bedrock.
Outwash is dominated by sand and gravel that have had
the muds removed. Grains are graded and stratified, abraded and rounded.
Glacial outwash is sediment transported by meltwater.
Loess (pronounced “luss”) is wind-transported silt.
Glaciers produce abundant amounts of fine sediment, which is picked up
and carried downwind.
Glacial lakes accumulate fine rock flour that settles out of suspension in deep lakes.
Glacial lake sediments often display seasonal varve
layers that reflect the finest silt and clay from frozen winter
months interlayered with coarser silt and sand from summer months.
Glacial sediments create distinctive landforms. These
include end moraines, terminal moraines, recessional moraines, drumlins,
ground moraines, kettle lakes, and eskers.
Cape Cod, Nantucket, Martha’s Vineyard, Block Island,
Long Island, and other prominent landforms in northeastern United States
formed at the end of the continental ice sheet.
Drumlins are elongate, tapered, and aligned hills of
molded glacial till that formed underneath the continental ice sheet.
They have an asymmetric form—steep up-ice, tapered down-ice—and are
common as swarms aligned parallel to ice-flow directions. They are
likened to a “field of swimming whales.”
Ice blocks calve off of glaciers
and become buried in sediment. When the ice melts, a kettle forms.
Hummocky knob-and-kettle topography typifies ground moraine.
If the water table is high, the
kettle will fill to become a kettle lake.
Eskers are long, sinuous ridges of sand and gravel.
They form as meltwater channels within or below ice. Channel sediment is
released when the ice melts.
The balance of accumulation and ablation determines
whether a glacier advances or retreats.
Glaciers form distinctive landforms, such as U-shaped
valleys, cirques, and horns. U-shaped valleys that fill with water
Deposition by glaciers produces distinctive landforms,
such as moraines, eskers, and kettle holes.
Continental Glaciation: Ice Loading and Rebound
Ice sheets depress the lithosphere into the mantle.
Slow crustal subsidence follows flow of asthenosphere.
This process continues slowly today. After ice melts,
the depressed lithosphere rebounds and the land rises.
Sea Level Changes
Ice ages cause sea level to rise and fall because
water is stored on land. Sea level was ~100 m lower during the last ice
age. Deglaciation returns water to the oceans and sea level rises. If
ice sheets melted, coastal regions would be flooded.
Sea level rise between 17 Ka and 7 Ka was the result of deglaciation.
Low sea level during the last ice age exposed continental shelves.
Prehistoric people migrated to North America from Asia
via the Bering land bridge.
Glacial Lake Agassiz covered 250,000 km2 (100,000 mi2)
and existed for 2,700 years. It drained abruptly when the ice dam
Large lakes occupied today's Basin and Range deserts.
The Great Salt Lake is a small remnant of the much larger Lake
Bonneville. Weather patterns were different during glaciation; the
American southwest was much wetter.
*Pluvial: a time period characterized by relatively high precipitation
The Pleistocene Ice Age: Glaciers
During the Pleistocene, several distinct ice sheets
formed. In several places, neighboring sheets came into contact.
The Pleistocene Ice Age: Life and Climate
All climate and vegetation belts were shifted southward.
Tundra covered parts of the United States, and southern states had forests like those in New England today.
Cold-adapted, now extinct, large mammals roamed
regions that are now temperate.
Pleistocene megafauna is the set of large animals that lived on Earth during the Pleistocene epoch and became extinct during the Quaternary extinction event.
Megafauna is a term used to describe an animal with an adult body weight of over 44 kg.
During the American megafaunal extinction event around 12,700 years ago, 90 genera of mammals weighing over 44 kilograms became extinct.
The Pleistocene Ice Age: Timing
Oxygen isotope ratios from marine sediments define 20
to 30 Pleistocene glaciations. Earth history has witnessed many ice
Causes of Ice Ages
Milankovic hypothesized that climate variation over 100 to 300 Ka is predicted by cyclic changes in orbital geometry.
Earth’s axis wobbles (precession) with a 23,000-year periodicity.
The angle of Earth’s rotational axis (obliquity) changes with a 41,000-year periodicity.
The shape (eccentricity) of Earth’s orbit around the
Sun varies with a 100,000-year periodicity. Ice ages may result when
cooling effects coincide.
Long-Term Cenozoic Cooling
Geologists have reconstructed an approximate record of
global climate for geologic time. Over the last 100 million years, Earth
experienced a warm climate at the end of the Mesozoic and climate
cooling since the Oligocene.
Will There Be Another Glacial Advance?
Are we living in an interglacial? Will ice return?
Very likely. Interglacials last ~10,000 years. It has been ~11,000 years
since the last one. A cool period (1300 to 1850) resulted in the Little
Ice Age in Europe. Today, a warming trend has caused glaciers to recede.
Earth’s climate changes without consulting humanity.
Little Ice Age in Europe from about 1300 to about 1850.
The weight of a growing continental ice sheet causes
underlying lithosphere to subside, then melting allows it to rebound.
During the Pleistocene (2.6 Ma to 11 Ka), ice sheets
advanced and retreated up to 30 times.
Advances and retreats during a given ice age are
controlled by Milankovitch cycles.
Greenland Ice Sheet